In addition, a Phase I dose escalation study utilizing 90Y-CYT-356 MAB was performed in 12 patients with hormone refractory prostate carcinoma. Biodistribution studies utilizing 111In-CYT-356 were performed one week prior to 90Y-CYT-356 administration. Fifty-eight percent of patients had at least one site of disease imaged following 111In-CYT-356. Both 111In and 90Y-CYT-356 were tolerated well without significant non-hematologic toxicity. Myelosuppression was the dose limiting toxicity. Only one patient developed a human antimouse antibody four weeks after treatment. No patient attained a complete or partial response based on PSA and/or radiologic criteria. Three patients had transient subjective improvement in symptomology of their disease.